Pergamon Armosphrric Envwonmenr Vol. 28. No. 19. pp. 3165-3169. 1994 Copyright 0 1994 Elscvicr Science Ltd Pnnlcd in Greal Brirain. All rights rcsemd 1352-2310/94 57.OO+O.C@ 1352-2310(94)00152-9 SEASONAL BEHAVIOR OF TROPOSPHERIC OZONE IN THE SAO PAUL0 (BRAZIL) METROPOLITAN AREA OSWALDO MASSAMBANI and FATIMA ANDRADE Department of Atmospheric Sciences, Institute for Astronomy and Geophysics, University of Sao Paulo, Sao Paulo, SP 01065-970. Brazil (Firs1 received I5 December 1992 and infinaljorm 16 November 1993) Abstract-This paper presents a study of the seasonal behavior of tropospheric ozone and its precursors in the Sao Paula Metropolitan Area as observed during 1987. The 0,. NO, NO,, NMHC, and meteorological data were collected at an air quality station in downtown Sao Paulo by the State Environmental Protection Agency (CETESB). The air pollutant measurements were related to both daily total insolation and the number of hours of insolation measured at the Sao Paulo University Climatological Station. Correlations between both radiation parameters and total daily integrated ozone amounts were Performed. The total number of sunshine hours was highly correlated to mean hourly ozone concentration values during each month of 1987. The seasonal behavior of NO, NO,, and NMHC was also studied. Two diurnal peaks in average NO concentration were observed. i.e. one in early morning and one in early evening; both were due to emissions from utban mobile sources. The magnitude of these peaks doubled in value during the winter months. Its diurnal concentration variation was inverse to that of the 0,; similar behavior was found for NO, and for NMHC. The data presented herein show the influence of solar radiation and of ozone precursors on photochemical smog formation in this tropical region. Key word index: Ozone, photochemical smog, Sao Paula. ozone precursors. INTRODUCTION The Sao Paulo Metropolitan Area (SPMA) is the largest industrialized region in the South Hemisphere, with about 15 million inhabitants. The climate in the SPMA shows a dry winter and a wet summer. August precipitation (winter month) is about 40 mm, while February (summer month) precipitation is on the order of 250 mm. Total annual accumulation is about 1700 mm. The are& has a large motor vehicle fleet and numerous heavy industries (CETESB, 1993). There are approximately 4.2 million automotive vehicles, 1.4 million use alcohol as fuel and 2.4 million use gasoline with 22% alcohol, and there are 326,000 diesel-fueled vehicles (Roman0 et al., 1992). The nitrogen oxides (NO,). carbon monoxide (CO), and hydrocarbons (HC) emitted by these fuels to the atmosphere are the precursors of ozone formation. When NO and NO, are exposed to sunlight, ozone formation occurs as a result of the photolysis of NO, Seinfeld (1986) sum- marizes such a reaction sequence, including the influ- ence of CO, and also discusses the role of non-methane organic carbon (NMOC). The specific photochemical reactions association with alcohol emissions are not yet available. The alcohol-fueled vehicles emission are composed of approximately 70% of ethanol, 20% hydrocarbons, and 10% aldehydes, this last group is composed of 85% acetaldehydes, 14% formaldehyde, and lessthan 1% acrilaldehydes. The formaldehyde is of great rel- evance, as it is an oxidation product of hydrocarbons. The photodissociation and oxidation of organic com- pounds (aldehdyes and hydrocarbons) is the best way to achieve NO oxidation without the destruction of 0, (Chock and Heuss, 1987; Chang and Rudy, 1990). Even in the particular SPMA atmosphere, ozone plays a very important role in photochemical smog mechanisms, According to Seinfeld (1986) and Murao et al. (1990) the most important sources of ozone pre- cursors are industrial process, the burning of oil, and secondary photochemical production. These anthro- pogenic sources are responsible for more than 95% of ozone concentrations. It has also been found that the 0, concentrations in the troposphere vary with sea- son, solar radiation, wind speed, temperature, and humidity (Kanbour et al., 1987). Pollutant concentra- tions depend also on advected emissions and on vertical mixing, and consequently on the local vari- ation of wind field produced parcel trajectories. This dependence makes the formulation of strategies for photochemical pollution abatement difficult in areas with frequent sea breezecirculations (Lalas et al., 1987; Wakamatsu et al., 1989; Liu et al., 1990; Murao et al., 1990), that is a usual situation in the region under study. This paper presents a study of the nature of 3165
Pnnlcd in Greal Brirain. All rights rcsemd 1352-2310/94 57.OO+O.C@
1352-2310(94)00152-9
SEASONAL BEHAVIOR OF TROPOSPHERIC OZONE IN THE SAO PAUL0 (BRAZIL) METROPOLITAN AREA
OSWALDO MASSAMBANI and FATIMA ANDRADE
Department of Atmospheric Sciences, Institute for Astronomy and Geophysics, University of Sao Paulo, Sao Paulo, SP 01065-970. Brazil
(Firs1 received I5 December 1992 and infinaljorm 16 November 1993)
Abstract-This paper presents a study of the seasonal behavior of tropospheric ozone and its precursors in the Sao Paula Metropolitan Area as observed during 1987. The 0,. NO, NO,, NMHC, and meteorological data were collected at an air quality station in downtown Sao Paulo by the State Environmental Protection Agency (CETESB). The air pollutant measurements were related to both daily total insolation and the number of hours of insolation measured at the Sao Paulo University Climatological Station. Correlations between both radiation parameters and total daily integrated ozone amounts were Performed. The total number of sunshine hours was highly correlated to mean hourly ozone concentration values during each month of 1987. The seasonal behavior of NO, NO,, and NMHC was also studied. Two diurnal peaks in average NO concentration were observed. i.e. one in early morning and one in early evening; both were due to emissions from utban mobile sources. The magnitude of these peaks doubled in value during the winter months. Its diurnal concentration variation was inverse to that of the 0,; similar behavior was found for NO, and for NMHC. The data presented herein show the influence of solar radiation and of ozone precursors on photochemical smog formation in this tropical region.
Key word index: Ozone, photochemical smog, Sao Paula. ozone precursors.
INTRODUCTION
The Sao Paulo Metropolitan Area (SPMA) is the largest industrialized region in the South Hemisphere, with about 15 million inhabitants. The climate in the SPMA shows a dry winter and a wet summer. August precipitation (winter month) is about 40 mm, while February (summer month) precipitation is on the order of 250 mm. Total annual accumulation is about 1700 mm. The are& has a large motor vehicle fleet and numerous heavy industries (CETESB, 1993). There are approximately 4.2 million automotive vehicles, 1.4 million use alcohol as fuel and 2.4 million use gasoline with 22% alcohol, and there are 326,000 diesel-fueled vehicles (Roman0 et al., 1992). The nitrogen oxides (NO,). carbon monoxide (CO), and hydrocarbons (HC) emitted by these fuels to the atmosphere are the precursors of ozone formation. When NO and NO, are exposed to sunlight, ozone formation occurs as a result of the photolysis of NO, Seinfeld (1986) sum- marizes such a reaction sequence, including the influ- ence of CO, and also discusses the role of non-methane organic carbon (NMOC). The specific photochemical reactions association with alcohol emissions are not yet available.
The alcohol-fueled vehicles emission are composed of approximately 70% of ethanol, 20% hydrocarbons, and 10% aldehydes, this last group is composed of
85% acetaldehydes, 14% formaldehyde, and less than 1% acrilaldehydes. The formaldehyde is of great rel- evance, as it is an oxidation product of hydrocarbons. The photodissociation and oxidation of organic com- pounds (aldehdyes and hydrocarbons) is the best way to achieve NO oxidation without the destruction of 0, (Chock and Heuss, 1987; Chang and Rudy, 1990). Even in the particular SPMA atmosphere, ozone plays a very important role in photochemical smog mechanisms,
According to Seinfeld (1986) and Murao et al.
(1990) the most important sources of ozone pre- cursors are industrial process, the burning of oil, and secondary photochemical production. These anthro- pogenic sources are responsible for more than 95% of ozone concentrations. It has also been found that the 0, concentrations in the troposphere vary with sea- son, solar radiation, wind speed, temperature, and humidity (Kanbour et al., 1987). Pollutant concentra- tions depend also on advected emissions and on vertical mixing, and consequently on the local vari- ation of wind field produced parcel trajectories. This dependence makes the formulation of strategies for photochemical pollution abatement difficult in areas with frequent sea breeze circulations (Lalas et al., 1987; Wakamatsu et al., 1989; Liu et al., 1990; Murao et al.,
1990), that is a usual situation in the region under study. This paper presents a study of the nature of
3165
3166 0. MASSAMBANI and F. ANDRADE
Total number of sunlight hours
6 7 Month
Temperature (Celsius)
1 2 3 4 5 6 7 a B IO 11 12
Month
Ozone concentration (ppm)
1 6 7 Month
Fig. I. Time evolution of insolation, temperature and ozone concentration lor 1987 in Sao Paula Metropolitan Area.
tropospheric ozone and its precursors in the special l-h maximum concentration air quality standard for ozone SPMA atmosphere and of their relationship to some adopted by the State of Sao Paula is 160 figme (81 ppb).
climatological characteristics of this tropical region. which should not be exceeded more than one day a year. The vear of 1987 deserves soecial attention as the above level was exceeded on 40 days. .
DATA RESULTS
Hourly ozone, NO. NO,, and NMHC data from the Parque D. Pedro station of CETESB in the center of Sao Paula were studied for 1987. The surface meteorological data
The daily evolution of temperature, hours of in-
(temperature, wind speed and direction, and relative humid- solation, and 0, concentration for 1987 are shown in
ity) were taken at the same site but the insolation data were Fig. 1. Values represent monthly averages for every
collected at the Climatological Station of the University of hour of the day. The peak monthly number of sunlight Sao Paula located 18 km southeast of the CETESB site. The hours were observed around noon time. as expected.
Seasonal behavior of tropospheric ozone 3167
For this work, we have considered this data as repres- entative of the whole Metropolitan area.
It is evident from the temperature diagram, that the presence of the three major maxima during the months of March, July-August, and November occur at about 1400-1500 LST with average temperatures above 27S’C. These temperature maxima also coin- cide with the maxima in the insolation diagram. It is important, however, to observe the time delay of about 3 h between the maximum input of short wave radiation and maximum air temperature.
The deviation of this insolation pattern from the predictable symmetric pattern of solar radiation at the top of the atmosphere is due to the presence of weather systems in the area and their associated cloudiness. The great effect of cloudiness for the months of February, May, and September can be clearly identi- fied by the decrease in the number of hours ofsunshine during that month. The measured daily solar radi- ation for February was only 410 calcm-‘d-l (the undepleted value at the top of the atmosphere is 970 cal cm - ’ d - ‘), for May the corresponding values were 220 and 600 cal cm-’ d- ‘, while for September they were 300 and 800 cal cm - ’ d - ‘.
In order to quantify the relationship of daily total solar radiation and daily total ozone concentration, a correlation analysis was performed. The r2 (Pearson correlation coefficient) computed for all months ranged 0.35-0.73. From these values, it was not pos- sible to obtain a consistent description of causality, even though such relationship is suggested from these diagrams.
The time evolution of ozone and its precursors is presented in Fig. 2, which shows the three month averages of the hourly values of ozone, NO,, NO and NMHC concentrations. It can be seen that the months of January-March have the greatest amount of ozone production. Peak ozone concentrations were observed around 1400-1500 LST, while minimum values were observed around 0700-0800 LST. The rate of ozone production was observed to be highest for the months of January and October, for which it was estimated at about 7.5 x 10m3 ppm h-‘. For the other months the rate was below 4.3 x IO-’ ppm h-i.
A slight secondary nocturnal peak was observed at around 0300 and 0400 LST, but these values were never greater than I2 ppb. During the early evening, the average ozone concentration was consistently below 5 ppb. Inspection of the hourly data for each day of the year revealed a wide range of events, showing cases of extreme concentration peaks that have caused alerts, as well as very low daytime concen- trations. Analysis of such case studies has clearly indicated the strong influence of weather systems entering the area.
Inspection of the nature of another atmospheric constituents in this urban atmosphere (such as NO), shows two peaks per day. The morning peak is seem to have formed around 0800 LST, while the afternoon peak initiated at about 1900 LST, and then decreased
slowly during the night period. These peaks are associ- ated with high traffic concentration during both rush hours in the SPMA. The NO peaks were observed to be always below 220 ppb. The minimum value during the sunshine hours was observed to coincide with the ozone peak (around 1400 and 1500 LST). It is also interesting to observe that while ozone reaches its minimum value in the early morning, associated NO concentrations reach their morning peak. This is an indication that the associated chemical reactions that took place before sunrise combined to the dilution and transport mechanisms have depleted the ozone con- centration that accumulated over-night in the surface atmospheric boundary layer.
Liu et al. (1990) and Samson (1978), among others, without looking at the behavior of precursor gases, have given an explanation for the nocturnal 0, peak that this is due to local atmospheric circulations. An inspection of individual events observed in SPMA have also indicated the effects of local circulations, but the effect of the chemical atmospheric reactions seems to have an even greater role in explaining nocturnal 0, behavior. The main sources of ozone precursors, the vehicles, start emitting the pollutants gases before the sunrise, and the NO emitted react with the oxygen, producing N02, but without the insolation, this can cause the depletion of 0,.
From the monthly NO data (not shown) we have also observed concentration peaks (reaching 250 ppb) for the months of June and July. During the months of September-November, the maxima only reached val- ues up to I50 ppb. These low values can be explained by the effects of intense convective mixing, as well as by the possible effects of rainfall scavenging. The winter (June, July) peaks are associated with the formation of a thermal inversion layer, causing an increase in precursor pollutant concentrations. The minimum NO concentrations around 1400-1500 LST are caused by depletion due to the emission and production of NO, and to the O3 formation.
Similar behavior was observed for the NO, concen- trations, i.e., this gas also shows two daily peaks (morning and evening) that are associated with local traffic, but that are also affected by the oxidation of NO. The morning peak was observed at about 1000 LST, about two hours delayed (in relation to the NO peak) due to oxidation reactions. The evening peak was observed at about 1900 LST. The data, however, indicate no delay in relation to the NO peak. The minimum NO1 concentration observed during sun- shine hours was also in good agreement to the ozone peak and the NO minimum. The NO, peaks were observed always less than 70 ppb. This illustrates its participation in the chemical reactions presented in Finlayson-Pitts and Pitts (1986). It is interesting to note that for this compound there is a faster rate of decrease after the evening peak, which seems to result from the depleting chemical reactions.
Although the higher concentrations of NO, (an ozone precursor) in the winter, the presence of more
Fig. 2. Time evolution of 0,. NO. NO:. NMHC for 1987. in Sao Paulo Metropolitan Area
insolation during the summer results in the presence of ozone peaks during the summer.
Similarly to the other compounds here discussed, NMHC shows two peaks that are in excellent agree- ment to the NO peaks. This is due to the fact that NMHCs are also injected into the atmosphere mainly by combustion from the SPMA mobile fleet. For each month, the averaged peaks did not reach 60 ppb. The minima observed value during the sunlight hours were never less than 20ppb; these minima happened in phase to the other non-O, compounds.
Both NO and NO, play the important role of catalyst for the oxidized forms of NMHC to produce ozone. The 0, formation process is nonlinear and under certain ratios of organic carbons to NO, (low ratios) additional NO will destroy 0,. This behavior of NO, reducing 0, is known as the “NO, inhibition effect” (Wolff and Korsog, 1992). It is interesting to note that the average behavior of 0, for the months of April-June and for July-September is very similar, in spite of the pronounced increase in NO,. However, NO concentrations only experience a change of about 10%. On the other side, NMHC concentrations were observed to be smaller for the months of July September in comparison to the months of April-June. August showed an anomalous high after- noon peak, which raised the average value for those
three months. Inspection of CO concentration meas- urements during August has also indicated the pre- sence of an intense afternoon peak. This effect was observed for only 2 h. and therefore is possibly due to an anomalous increase in local source emissions. For the months of April-June. both peaks (morning and evening) were observed to have about the same magni- tude and to be higher than those during July- September. The solar radiation input into the SPMA atmosphere for the months of April-June measured a total of 295 cal cm-’ d- ‘, while for the months of July-September it measured 320 cal cm-’ d- ‘. As- suming that for those months the SPMA atmosphere was exposed to about the same solar radiation intens- ity. the decrease in NMHC has not affected 0, concentration, in agreement with predictions from the laboratory model studies of Chock and Heuss (1987).
In order to describe the seasonal nature of the tropospheric ozone in the SPMA, its behavior was studied in comparison to precursor gases (NO, NO?, and NMHC) and to meteorological parameters. An analysis between daily total Ozone concentration and daily total solar radiation has indicated monthly r’
Seasonal behavior of tropospheric ozone 3169
values ranging only 0.35-0.73, even though the pat- tern agreement is excellent. It was observed that a slight secondary peak is present around 0300 and 0400 LST reaching values not greater than 12 ppb. The daytime peak, however, was up to 50 ppb, and that the months of January-March had the greatest produc- tion rate.
The NO concentrations showed two peaks during the day in association to high traffic concentration during rush hours. At the same time that ozone reaches its minimum value in the early morning. NO concentration reaches its peak, indicating that chem- ical reactions have depleted the ozone accumulated overnight in the surface atmospheric boundary layer. The same peak behavior of NO was found for N02, but about two hours later than the NO peaks. The NMHC also shows two peaks in excellent time agree- ment to the NO peaks, as an indication of the same source origin. This study has thus shown the signific- ant influence of the solar radiation and of ozone precursors on photochemical smog formation in this SPMA tropical atmosphere, with its unique alcohol- fuel-based mobile emission mix.
Acknowledgements-The authors acknowledge CETESB for providing the air quality data, and Professor Robert Born- stein for his valuable comments during the writing of this paper.
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